Most (if not all) televisions work using three colored pixels; red, green and blue. Combine all three, and you get white. How does the television make black? It would seem simple enough to say that you simply don’t turn any of the pixels on, but I have noticed that the ‘black’ you see when the television is off looks very different from the ‘black’ you see on the top and bottom of a widescreen movie. So how is my television creating black? What about grey?
The black you see when your TV is on is actually somewhat brighter than what you see when it’s off. The red, green and blue pixels are illuminated at a very low level. It’s actually a very dark gray. That answers the second part of the question, too. In order to make any shade of gray, you must illuminate the red, green and blue pixels at the exact same intensity.
Black parts of the screen only appear dark in contrast to the bright parts, in reality, they can’t be any darker than they are when it is turned off.
Easy; they have a dark light mounted in the back, that shines darkness on the parts that are supposed to be black. That is why LCD tv’s don’t have very good blacks - not enough room for the darklight.
So that’s what that extra filament in the lightbulb in the bathroom is: the dark filament. When I turn the light “off” I’m actually turning the dark on…
No wonder my hydro bill is so high…
The same way they print the white bits in newspapers.
Sunspace I bet you get though a lot of white toner too 
[QUOTE=Small Clanger]
The same way they print the white bits in newspapers./QUOTE]
This doesn’t explain why the black on a widescreen movie is different from the black when the television is off.
This must be to do with the contrast in the picture, “off” bits of screen will look blacker in comparision with the bright sections (we need a vision specialist on this). If you crank up the brightness even “black” areas will turn out to be very dark grey/blue.
It is possible that in wide screen format the masked bits of screen are in fact off (now we need a TV expert) so turning up the brightness won’t effect top/bottom sections and they will always be darker than the picture.
I’ve not seen this but a friend complained about artifacts from signal compression in space based programs (Trek, B5 etc) where you’d see areas of different shades of “black” where there was a lot of space on screen.
From How Stuff Works:
I can accept this explanation, but it still doesn’t explain the difference between the-tv-is-off black and the-tv-is-on black. Is it simply a case of reflection of some sort from the energized pixels ‘leaking’ low levels of power into the non-energized pixels? I do not agree with Small Clanger’s explanation that it is simply a matter of contrast. I believe that in a totally dark room with a television showing a totally black image (as it might before the credits but after the film) there will still be more visible light than with the television off.
Look at this illusion. The bar in the middle is the same shade of grey throughout:
http://www.cut-the-knot.org/Curriculum/Geometry/Contrast.shtml
It’s because of the NTSC format. Absolute black is used for synchronization of the frame, so the signal for black is sent as a very dark grey.
Y’all ain’t listenin’ - the expected result of an illusion due to contrast is the opposite of the OP’s experience - and I have the same experience. Even an entirely black TV, if turned on, is emitting some light, while a TV that’s turned off is not. Some may be due to the phosphors glowing from previous excitation - note that a TV will glow for a bit after it’s turned off. But that’s not a complete answer either - even movie letterboxing (areas of screen that, thus, are black for long periods) appears to me to emit SOME light, while a TV powered off emits none.
Well, freido answered the question in post #2. The black is actually a very dark grey. Yes, it’s different than the black you get when the TV is turned off. No, it’s not actually black.
Okay that sounds reasonable enough. My next question then is why bother with a very dark gray? Why not just leave the black un-illuminated?
See post #12.